1. Field of the Invention
The present invention relates to a liquid crystal display device, in particular to a plasma addressed liquid crystal display device (PALC). The present invention also relates to a dielectric sheet for separating a plasma switching section from a liquid crystal cell section of a PALC, and a method for producing the same.
2. Description of the Related Art
FIG. 26 is a perspective view of a conventional plasma addressed liquid crystal display device (PALC), and FIG. 27 is a cross sectional view thereof. The conventional plasma addressed liquid crystal display device will now be described with reference to FIGS. 26 and 27.
The plasma addressed liquid crystal display device includes two sections: a plasma switching section 1a and a liquid crystal cell section 1b. The plasma switching section (also referred to as a plasma cell substrate) 1a includes a glass substrate 4, a dielectric sheet 6, and a plurality of partition walls 5 formed between the glass substrate 4 and the dielectric sheet 6, and a plurality of plasma discharge channels (i.e., plasma generating region, plasma channel, or plasma cell) surrounded by the glass substrate 4, the dielectric sheet 6, and the plurality of partition walls 5. The liquid crystal section 1b includes a liquid crystal layer 7 and a counter substrate (also referred to as a color filter substrate in the case where it includes a color filter layer) 1b'. The liquid crystal layer 7 is interposed between the dielectric sheet 6 and the counter substrate 1b'. The counter substrate includes a glass substrate 10 and a plurality of strip electrodes 8 on a side of the liquid crystal layer 7. The counter substrate 1b' may include a color filter layer 9 having R, G and B color filters for conducting a color display.
In the plasma cell substrate 1a, alternating strips of anode electrodes 14 and cathode electrodes 12 are formed on the glass substrate 4 having a thickness of about 2 mm. The partition wall 5 for separating a plasma discharge channel 17 are formed on each of the anode electrodes 14. Prior to the formation of the anode electrodes 14 and the cathode electrodes 12, an underlying film 13 of, for example, an SiO.sub.2 -type material is formed on that surface of the glass substrate 4 on which the anode electrodes 14 and the cathode electrodes 12 are to be formed.
The dielectric sheet 6 for separating the plasma discharge channels 17 from the liquid crystal layer 7 is formed on the partition walls 5. Conventionally, a thin plate of glass is used as the dielectric sheet 6. After being evacuated into a vacuum state, each plasma discharge channel 17 between the partition walls 5 is filled with a rare gas (such as He and Ne) containing a small amount of Hg, in order to cause a plasma discharge in the plasma discharge channel 17.
In the counter substrate 1b', a black matrix 9a and a color filter layer 9 are formed on a glass substrate 10. On the color filter layer 9, strips of signal electrodes 8 of indium tin oxide (ITO) are formed substantially perpendicular to the partition walls 5.
Alignment films 15 and 16 (not shown in FIG. 26) are respectively applied on the opposing surfaces of the dielectric sheet 6 and the color filter layer 9 having the signal electrode 8 thereon. Then, the alignment films 15 and 16 are rubbed. The plasma cell substrate 1a and the counter substrate 1b' are attached to each other with a spacer (not shown) being provided either on the plasma cell substrate 1a or on the counter substrate 1b' for maintaining a prescribed cell gap (i.e., a thickness of a liquid crystal layer). At this time, the plasma cell substrate 1a and the counter substrate 1b' are attached to each other so that the respective rubbing directions are substantially perpendicular to each other. The gap between the plasma cell substrate 1a and the counter substrate 1b' is filled with a liquid crystal material, whereby the liquid crystal layer 7 is formed. Polarizing plates 3 and 11 are provided to the respective outer surfaces of the glass substrates 4 and 10 such that their polarization axes correspond to the respective rubbing directions. Accordingly, the polarization axes of the polarizing plates 3 and 11 are substantially perpendicular to each other. A surface-emitting backlight 2 is provided on the plasma cell substrate 1a side.
(Operation principle of plasma switching section)
The operation principle of the plasma switching section will now be described with reference to FIGS. 28 and 29. It should be noted that time periods (1) to (6) in FIG. 29 correspond to (1) to (6) in FIG. 28, respectively.
In the plasma addressed liquid crystal display device (PALC), the plasma discharge channels 17 in which a plasma discharge is caused are subjected to switched line-sequential scanning, and a data signal or an image signal is applied to the signal electrodes 8 in synchronization with the scanning, whereby display driving is conducted. When the anode electrodes 14 are connected to the ground and a negative pulse voltage is applied to the cathode electrodes 12, a plasma discharge occurs within the corresponding plasma discharge channel 17 (FIG. 28, (1)), and carriers (space charges) (ions/electron pairs) for writing data are produced. Accordingly, the plasma discharge channel 17 is rendered at the same potential as that of the anode electrode 14. Furthermore, an interface potential is produced at that surface of the dielectric sheet 6 which faces the plasma discharge channel 17, whereby a virtual electrode (not shown) is formed. When the application of the negative pulse voltage is discontinued after a prescribed time period, the plasma discharge is finished. However, the virtual electrode still remains at the same potential as that of the anode electrodes 14. When a data voltage corresponding to the data signal or the image signal is applied to the signal electrodes 8, the data voltage is divided according to the capacitance division ratio of the dielectric sheet 6 to the liquid crystal layer 7, whereby a prescribed image signal is applied (written) to the liquid crystal layer 7 (FIG. 28, (2)). When the plasma discharge is finished, the carriers will disappear over time, and the plasma discharge channel 17 will return to an insulating state. The accumulated charges corresponding to the image signal applied (written) to the liquid crystal layer 7 are retained until the next discharge occurs in response to the application of a negative pulse voltage (FIG. 28, (3)). By conducting the series of operations described above, display data corresponding to a single line is output from a liquid crystal driver to the signal electrodes 8 on a plasma discharge channel by plasma discharge channel basis. Thus, the data corresponding to a single line is written to the liquid crystal layer 7 at one time. In order to prevent degradation of the life of the liquid crystal material, the write operation to the liquid crystal layer 7 is conducted by alternating-current driving of the anode potential. Accordingly, data is written to the liquid crystal layer 7 with a polarity of the data voltage being inverted on a line by line basis. FIG. 28 further shows a plasma discharge (4), a data write operation (5) and a data retaining operation (6) in the case of the write operation conducted with polarity inversion. The operation principle in the case of (4), (5) and (6) is the same as that in the case of (1), (2) and (3) except that the polarity of the data voltage is inverted in (4), (5) and (6). The series of operations described above is sequentially conducted on the plasma discharge channel by plasma discharge channel basis, whereby an image corresponding to a single frame is displayed.
(Dielectric sheet)
The dielectric sheet 6 will now be described.
In the plasma addressed liquid crystal display device (PALC), light from the backlight 2 provided on the plasma cell substrate 1a side is modulated through a twisted nematic (TN) liquid crystal. Since the plasma addressed liquid crystal display device utilizes such a light modulation effect, the dielectric sheet 6 which is located therebetween should have an excellent transmission of visible light. Moreover, in order to prevent such disadvantages as crosstalk produced in the plasma-scanning direction, it is necessary that insulation is established between induced plasma-channel lines. Glass has been suitable as a material which satisfies these conditions. Japanese Laid-open publication No. 4-313788 proposes a method for solving these problems. According to this proposed technology, conductors are provided within a dielectric sheet so as to correspond to the pixels, and a transparent electrode pattern is formed on a pixel by pixel basis on that surface of the dielectric sheet which is in contact with a liquid crystal layer.
Conventional plasma addressed liquid crystal display devices including the above-mentioned proposed technology have the following problems:
(Surface stability of interface potential of virtual electrode)
When a plasma discharge occurs, an interface potential of the virtual electrode is produced at that surface of the dielectric sheet 6 which faces the plasma discharge channel 17, as described above. During the plasma discharge, the spacial charge distribution in the plasma discharge channel is not uniform. Therefore, the interface potential of the virtual electrode is adversely affected by the plasma discharge potential, making the surface charge distribution of the virtual electrode non-uniform and unstable. As a result, the surface uniformity of the voltage application to the liquid crystal layer 7 corresponding to the pixels is degraded, causing non-uniformity of the luminance. Consequently, the display becomes undesirable.
(Voltage applied to liquid crystal)
Voltage application to the liquid crystal layer is conducted by applying a voltage between the data electrodes (signal electrodes) of the counter substrate and the anode electrodes through the dielectric sheet of the plasma cell substrate. According to a capacitive coupling model, a voltage V.sub.LC applied to the liquid crystal layer is given by the following equation: EQU V.sub.LC =V.multidot..epsilon..sub.G.multidot.d.sub.LC /(.epsilon..sub.G.multidot.d.sub.LC +.epsilon..sub.LC.multidot.d.sub.G) (1)
where V indicates a data voltage, .epsilon..sub.LC indicates a dielectric constant of the liquid crystal layer, d.sub.LC indicates a thickness of the liquid crystal layer, .epsilon..sub.G is a dielectric constant of the dielectric sheet and d.sub.G indicates a thickness of the dielectric sheet.
In the case where the following exemplary values: .epsilon..sub.LC =6.7, d.sub.LC =6.0 .mu.m, .epsilon..sub.G =5.8and d.sub.G =50 .mu.m are substituted for the above equation (1), the following equation is obtained: EQU V.sub.LC =0.094 V (2)
It should be noted that the exemplary values mentioned above are values of a typical liquid crystal and a thin plate glass (dielectric sheet). Since the dielectric constant (.epsilon..sub.LC) of the liquid crystal varies depending upon the voltage, the coefficient 0.094 of the data voltage V in the above equation (2) is a function of the voltage V.sub.LC, making the calculation complicated.
(Crosstalk)
According to the operation principle of the plasma addressed liquid crystal display device, a data voltage corresponding to an image signal is applied to the liquid crystal layer 7 through the dielectric sheet 6. Accordingly, when the image signal is applied (written) to the liquid crystal layer 7, a charge pattern corresponding to the image signal is produced at the virtual electrode. Due to the thickness of the dielectric sheet 6 or the like, this charge pattern produced during the write operation expands in a lateral direction during the charge retaining operation. This lateral expansion adversely affects the adjacent pixels, causing crosstalk. Crosstalk reduces the pixel resolution and causes undesirable color mixture, thereby degrading the color reproduction capability.
(Data driver)
A liquid crystal driving voltage should be normally about 5 V. A data voltage of about 53 V is required in the above-mentioned example. Accordingly, a data driver consumes a large amount of power for a driving operation. Moreover, a semiconductor layer capable of withstanding a high voltage is required, making the driver expensive.
(Distortion of thin plate glass)
When the liquid crystal material is introduced a pressure on the liquid crystal layer side is 1 atm, while the rare gas within the plasma discharge channel has a low pressure of several tens of Torr. Moreover, the thin plate glass (dielectric sheet) having a small thickness is distorted between the partition walls, causing defective liquid-crystal orientation. Accordingly, the cell thickness varies within a pixel, whereby the retardation is shifted from its design value. As a result, the brightness and viewing-angle characteristics become undesirable.
(Handling of thin plate glass)
It can be seen from the above-mentioned relationship between V.sub.LC and V that the thinner glass is used, the higher data voltage is applied to the liquid crystal layer. This is advantageous in terms of the data voltage. However, reduction in the thickness of the glass is practically limited in terms of its strength. More specifically, the thinner glass is more fragile, causing a reduction in yield. This is not preferable from the production point of view.
(Numerical aperture)
The position of each conductor provided within the dielectric sheet corresponds to a pixel. Therefore, the numerical aperture of the pixel is reduced by the cross sectional area of the conductor.